Archive for the ‘Green Energy’ Category

Optimizing CPV Systems

2011/09/16

Optimizing CPV Systems

Concentrating Photo-Voltaic (CPV) systems have some competitive advantages, for example,

  • CPV uses land efficiently, i.e., MW/acre is excellent for CPV
  • CPV does well when ambient temperatures are very high, e.g. in excess of 110 degrees Fahrenheit (43°C).
  • CPV needs no water to operate and very minimal water for maintenance work.
  • CPV seems to have very minimal ecological impact – better than wind and other forms of solar power generation.

Thus CPV works well in deserts where there is a nearby connection to the grid, and where the regional electric company can take all the power that a CPV farm generates and can put up with no power at night and slightly irregular power due to occasional clouds.  (Today CPV systems tend not to have energy storage mechanisms to smooth out the energy delivery to the grid.) This defines the CPV “niche.”

CPV systems have the disadvantage in that they are complex.  Starting with three layer multi-junction photovoltaic cells, which are expensive in their own right and somewhat complicated to wire into arrays and panels, they need a Fresnel lens to concentrate sunlight onto (or more precisely into) these cells.  They require that the sunlight rays be orthogonal to the plane of the panel, and this means that the panel must track the sun using a two axis system that adjusts both the azimuth and polar angles of the panel.  Large panels need to be mounted relatively high so that the panel clears ground objects as it “tracks” during the day.  A robust structure is needed to support the system weight and the  forces of wind on the system.  Depending on the locality, service roads and fencing might have to be installed.  Finally each CPV structure needs an inverter to convert DC to AC, and a connection to the grid.  The cost of each of these components is, of course, the first point of attack in optimizing CPV systems.  These are the capital costs that need to be optimized.

There are of course non-trivial, non-capital installation, calibration, maintenance, and operational costs.

The capital, installation, and operational costs of a CPV system can be mitigated in the near future by a favorable regional government-dictated Feed-in Tariff (FiT), which guarantees a grid connection, a long term contract, and rates that take into account these costs and that guarantee an operational profit. Italy’s FiT is particularly advantageous, since it has a special tariff table for CPV. [1]  Now if competition weren’t enough motivation for CPV vendors to drive costs down, most Feed-in Tariffs reduce the guaranteed rates by a small percentage each year to motivate the vendors to take advantage of technology improvements and reductions in operating costs.

As mentioned in an earlier post on Levelized Cost of Energy (= a system’s lifetime future costs divided by the lifetime energy it will produce) there are many components to lifetime costs, each of which needs to be consciously and systematically driven down by the CPV vendor. What is needed here is a company-specific model for these costs and a continuous improvement program to drive each cost component down.  NREL’s System Advisor Model (SAM) is a very nice start (and its earlier versions motivated the definition of LCOE), but a company needs more detail to optimize costs.

For example, a CPV system is particularly sensitive to being aligned precisely (to a small fraction of a degree) to the sun.  This necessitates high precision in its two axis tracking system.  In addition to its additional capital cost, a tracker has additional installation and maintenance costs.  Mounting a tracker on a high pedestal created additional weight and hence wind force factors.  Each of these details (accuracy, capital cost, installation cost, maintenance cost, weight, and required structural support to address wind forces) must be optimized by the CPV vendor.

LCOE isn’t the only useful metric for a CPV vendor to consider.  Now marketing folks and the press like big numbers, and hence the peak power or Watts-Peak (Wp) makes a lot of headlines. It is the maximum power that can be generated by the system.  Sometimes this number is tempered (no pun intended) by NREL’s Standard Test Conditions (STC) to get a “name plate” rating.  Probably a little more interesting is the Energy to Peak Power ratio.  Define:

SI = Site Irradiation (kWh/m2/yr)

RI = Rating Irradiance (kWp/m2)

PCE = DC to AC power conversion efficiency

ATE = Average temperature efficiency

EPP = Energy to Peak Power Ratio = SI*PCE*ATE/RI

The Site Irradiation, SI, is really the actual energy produced by the CPV system per unit area, say over a year.  Since the energy produced varies with time, this needs to be a sum over time-intervals small enough so that the energy produced in each time-interval is approximately constant.  Similarly the ATE varies with time, since the device will heat up during the day causing increased degradation.  One then uses time-intervals small enough so that the temperature during the time-interval is approximately constant.  PCE is just the inverter efficiency.  The EPP ratio is a little flawed in that the numerator has the factor of hours per year in it.  A mathematician would divide it out, but the industry tends to leave it in.  Oh well, …

While LCOE has most of the EPP terms in its ultimate calculation, it is none-the-less instructive to track this ratio as well as to optimize each term.  Note that the Energy to Peak Power ratio is a function of locality as is LCOE.

The significant improvement in solar cell efficiencies has driven CPV’s LCOE down in past years due primarily to increased energy production.  PV, on the other hand, has had its capital costs driven down dramatically by massive Chinese government investments.  This has driven down fixed thin film PV’s LCOE.  The net result of these two trends is to make thin film PV more attractive EXCEPT in the niche described earlier for CPV dominance.  This race of technology and manufacturing improvements will, of course, continue.  Unfortunately, the three major CPV vendors, Amonix, Concentrix, and SolFocus are all going after smaller numbers of large “utility sized” sites.  This doesn’t lend itself to cost or manufacturing efficiencies as compared to the goal of putting thin film PV on every rooftop in the world. CPV vendors will need to invest to compensate for this.  It will be a challenge.

-gayn

[1]  http://www.mwe.com/index.cfm/fuseaction/publications.nldetail/object_id/1f69afd9-2855-467f-a2cb-0ef9c98ad128.cfm

[2] “LCOE For Concentrating Photovoltaics (CPV)” by Warren Nishikawa, Steve Horne, Jane Melia, warren_nishikawa@solfocus.com, SolFocus Inc., 510 Logue Ave., Mountain View, CA 94043 International Conference on Solar Concentrators for the Generation of Electricity (ICSC – 5), November 1619, 2008, Palm Desert, CA USA (www.icsc5.com )

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Levelized Cost of Energy (LCOE)

2011/09/03

Levelized Cost of Energy

For any power generating system, one can compute the “levelized cost of energy” (LCOE) over the predicted lifetime of the system.  It is the ratio of the present value of the total cost of operation or ownership (TCO) to the total energy generated (TEG) over the predicted lifetime of the system.  This ratio, LCOE =TCO/TEG, looks simple, but the devil is in the details (as my mother used to say.)

LCOE does have a simple interpretation.  First note the units:  Today’s cents, dollars, Euro’s, etc. are in the numerator, and kilo-watt-hours (or mega-watt-hours) are in the denominator.  Usually it is cents per kWh or dollars per MWh.  If you want to build a power plant, you definitely want to sell the power you put onto the grid at a price greater than your LCOE, or you will lose money.

You can also compare the cost efficiency of various systems, and indeed, of various types of systems.  Here’s a table of such from a respected government source [1]. It indicates how LCOE can vary by region.  E.g. Transportation charges for coal, amount of sunlight and incidence angles for solar, amount of wind for windmills all vary by region.  However interesting these tables are, there is no guarantee that the LCOE was calculated fairly in each case.  Thus before making any decisions based on LCOE numbers, be sure you really understand how they are computed.  The text after the table is a start.

Regional Variation in Levelized Cost of New Generation Resources, 2016.

Plant Type Total System Levelized  Costs
(2009 $/MWh)

Minimum

Average

Maximum

Conventional Coal

85.5

94.8

110.8

Advanced Coal

100.7

109.4

122.1

Advanced Coal withCCS

126.3

136.2

154.5

Natural Gas-fired
Conventional Combined Cycle

60.0

66.1

74.1

Advanced Combined Cycle

56.9

63.1

70.5

Advanced CC with CCS

80.8

89.3

104.0

Conventional Combustion Turbine

99.2

124.5

144.2

Advanced Combustion Turbine

87.1

103.5

118.2

Advanced Nuclear

109.7

113.9

121.4

Wind

81.9

97.0

115.0

Wind – Offshore

186.7

243.2

349.4

Solar PV1

158.7

210.7

323.9

Solar Thermal

191.7

311.8

641.6

Geothermal

91.8

101.7

115.7

Biomass

99.5

112.5

133.4

Hydro

58.5

86.4

121.4

Source:
Energy Information Administration, Annual Energy Outlook 2011,
December 2010, DOE/EIA-0383(2010)

For example, this table indicates that wind is comparable to natural gas.  This surprises me.  Also note how much more offshore wind costs.  This is particularly surprising given the large investment that is under weigh along the east coast of the U.S.  Hydro is a winner, but as my son Sam pointed out to me, it is unlikely that these numbers compute the damage done to the fishing industry (esp. salmon fishing) by these dams (environmental damage can be modeled as uninsured costs, see below).  This is another devilish detail.

Now, let’s look at the devilish details:

First the predicted or expected lifetime of the system assumes excellent maintenance and reasonable upgrades.  At the beginning of the lifetime, there is the original (capital) cost of construction and of the hook-up to the grid, and at the end of the lifetime there is the decommissioning and waste management of all the remaining materials.  The original cost needs to include things like access road improvements, right of way purchases, the installation’s fair share of grid improvements necessary for the hookup, etc.  Some towns, knowing that all the construction vehicles passing over its roads will wear on those roads, may want compensation in order to repair or refurbish them after the construction.  This should be included in the original cost.  Here the cost of financing needs to be carefully included.  The decommissioning cost is often (fraudulently in my opinion) excluded.  It must include tear down costs, land fill costs for the debris, and the safe storage of chemicals, fuels, and radioactive material.  It might include, instead, the cost of a complete refurbishing of the system to make a totally new power generation plant.  In this case, one must fairly separate the decommissioning and waste management cost of the old system from the original cost of the new system.  The residual value of the old system needs to be subtracted from its TCO.  Similarly, if waste steel is recycled, its residual value needs to be subtracted from the old system’s TCO.

Note:  Some people believe that if nuclear power plant LCOE included the total decommissioning and cost of nuclear waste removal and storage, then nuclear simply wouldn’t look very attractive economically.  As a boy, I used to think that waste nuclear fuel should be put into rockets and shot into the sun.  Sadly, this quite reasonable idea is not feasible economically.

Now during the lifetime of the system, there is a lot of operational cost, maintenance, repairs, and upgrades.  There are many of these.  Here are some:

  • Land lease costs.
  • Labor, travel, fuel, materials, etc. costs for operations and maintenance.  Note that some fuel prices, e.g. nuclear and petroleum, might have difficult-to-predict cost variations due to political considerations.
  • Large, infrequent upgrades or replacements.  A ten year replacement for inverters is often mentioned.
  • Insurance costs.
  • Property tax costs (but not income tax as that speaks to profitability not cost).
  • Utility costs, e.g. water, sewage, network communications, telephone charges.
  • Uninsured liability, theft, vandalism, disaster, and ecological damage costs.
  • Future green-house gas GHG charges.

These all have to be estimated over time and space (region), the present value of these costs needs to be calculated, and added up, we get the total cost of ownership, TCO.  As mentioned above, these costs will vary by region and of course over time. Some costs are correlated to an appropriate conflation index, and others need to be modeled.  All assumptions need to be carefully documented.

Finally, the economic assumptions for the present value calculation of all these costs must be documented.

Let’s turn to the denominator, the total energy generated over the predicted lifetime of the system.  For very stable sources of energy, e.g. nuclear, hydro, coal, natural gas, etc., one can reasonably assume a policy of steady consistent energy production, e.g. a certain number of kWh per day.  For wind, solar, geothermal, tidal, etc, there are simple models for energy generated, say per year, and there are more detailed models that would depend on weather and warming trends.  These latter can be quite sophisticated.  There are some additional subtleties to consider.  For example, not all of the power rating of the system gets onto the grid as energy, i.e. as electricity.  Some of this difference is standard due to energy conversions and efficiency of equipment.  For wind and solar farms, not all of the equipment is 100% operational; for example, some of it may be down for routine maintenance or repairs.  (PV systems need to be cleaned regularly, and the system will degrade as its solar cells or lenses slowly get dirty.)  Some reasons are more subtle, e.g. time shifting energy production via the use of an energy storage mechanism (CAES, liquid salt, MgH2, batteries, etc.)  Conversion both to and from the storage mechanism produces an energy loss.  Thus the algorithm for time shifting needs to be considered in the calculation for LCOE.

Given that some of the variable components of both TCO and TEG are “random”, i.e., depend on random events such as weather and politics over time, it is often appropriate (and easier) to make assumptions on these random event distributions and run Monte Carlo simulations for the calculations.  Argonne Laboratories has written a paper on this.

Note that the calculation of LCOE avoids (except for the reasons for time shifting) what the electric companies will pay for energy put onto the grid.  LCOE is one massive average over a long (20-30 year) lifetime.  Electrical rates are quite another thing.

Electrical rates vary across the day, with utilities charging commercial enterprises more for electricity during “peak hours” than during “off hours”.  The differences can be considerable.  Solar systems generate most of their power during peak hours.  If a solar system is directly substituting for utility company power during peak hours, then the value to the owner of that system will be, during these hours at least, equal to the peak rate the electric company charges. This type of logic is not directly factored into LCOE calculations; however, a favorable power provider agreement (PPA) between the generator owner and the utility can improve the deal for financing charges.  Residential and most commercial installations of solar power will not get a favorable agreement with the utility company.  Per the California Public Utility Commission, excess electricity generated and not used, i.e. put back onto the grid, is to be compensated annually at the average spot rate for the year – better than nothing, but far less than peak rates and not a real incentive to install solar power on your roof for the purpose of making money by selling the excess energy.

As explained before in these notes (here) irregular sources of utility scale energy such as wind and solar may well be more profitable with local energy storage.  This would allow a base level of energy to be put onto the grid, and also some additional energy for peak times.  To model this, a revenue model needs to be created in parallel with the LCOE model.  This can be done in a spreadsheet, but it can also be done with Monte Carlo methods.  These models need to be part of the pro forma economic analysis done at site selection and system design time.  The PPA negotiation with the utility company needs to conform with the model results.

Finally, NREL publishes a simple LCOE calculator here, which can be used to test any LCOE calculation for reasonableness.

-gayn

[1] Levelized Cost of New Generation Resources in the Annual Energy Outlook 2011

Smart Appliances and Energy

2011/08/30

Smart Appliances and Energy

With more and more household appliances using microprocessors to add “smart” features, their energy consumption can become an issue.  In fact, even the dormant state of these appliances can consume significant amounts of electricity.

One of the worst offenders is the now ubiquitous cable TV box, of which my house has half a dozen.  These use power to retain state.  This is superficially reasonable, since the catalog of the next several days of TV shows is part of that state (as is all the personal settings.)  Downloading this catalog takes several minutes, which would be annoying for most consumers, who want “instant” access to their TV show listings.  As these boxes contain Internet connections, which take time to establish a connection, there will be a tendency to use power to maintain that connection 24 hours per day.  Here some design work is required to minimize energy consumption while maintaining fast reaction times when the TV is turned on.

Now we move to refrigerators.  One can imagine quite a number of applications for a microprocessor in the refrigerator.  Recipes from the Internet that pop up when a food bar code is scanned, or entered into the computer, but probably a more useful feature would be an inventory of what is being stored (fresh or frozen).  While storing an item, swipe its bar code so that the system knows when you first stored it and hence how old it is later.  Enter its expiration date and location in the refrigerator.  Coming back to recipes, how about a list of recipes that can be made from ingredients in the refrigerator?  Nice!  Even nicer would be to access this inventory via cell phone when at the market.  “Do I have any fresh cilantro at home?”  My current refrigerator simply beeps at me with the door is left open, but why not send my cell phone a message?  Of course, access to the Internet allows access to video based cooking instructions to be downloaded and played.  Want to see that technique to whip up a great soufflé?

Smart stoves and ovens have a different play when hooked up to the Internet.  My favorite would be to warm up my oven as I’m heading home from the market.  Integrated temperature probes are popular with microwaves, but should be integrated into ovens as well.  Sensors that turn off that stove when the pot has boiled away all liquid would be a nice safety feature.  OK, all these features are nice, but what is the cost of the energy?  Again, some design work is needed to minimize energy consumption and still to maintain fast power ups.

Refrigerators have another option that was discussed briefly in an earlier post.  When it is cheaper to use electricity at night, a refrigerator can wait until night time to make the system extra cold so that it can more easily ride out usage during the day.  Note that if the building has solar power to augment the utility provided electricity, the solar power is “free” during the day and expensive at night.  Thus, it makes sense to keep the system extra cold during the day and to almost power off at night.

My current microwave’s defrost features are terrible.  Look, I want to defrost a steak quickly without partially cooking it.  I also don’t want to wait an hour defrosting a big chicken.  The smarts to do this are SMOP (“simply a matter of programming”).  My microwave gives me a ridiculous choice for defrosting options.  How about getting the right option off the Internet by scanning its bar code?  Also, a microwave should probably be hooked up to a digital scale to weigh things, but putting the scale into the microwave itself has its charm.  A lot of these features can be implemented without using too much excess energy.  In fact, considerable energy can be saved, simply by not overcooking things!

Now, who is going to invent the “green” robot that does all my cooking for me?

Of course the moral here is that smart appliances are coming, and I hope that industry will embrace saving energy.

-gayn

Storing Energy as MgH2

2011/08/29

Storing Energy as MgH2

With wind and solar power generation, it is often desirable to store energy locally rather than to put it onto the grid.  For example,

  • Sometimes the rates paid by the electric company are not favorable, e.g., at non-peak times, or are even negative.
  • Usually the electric company wants a “base load” to be provided, i.e., a minimum amount of electricity that goes onto the grid 7×24.  Neither wind nor solar can do this without burning another fuel such as natural gas or hydrogen.
  • A capital cost reduction can be partially realized if the generation system is designed NOT to generate more than a certain maximum level of electricity, with the rest being stored to address the base load issue.  The reason for “partially” is that the capital cost of the storage system needs to be factored into the total cost.

One storage possibility is to store the energy as hydrogen gas using electrolysis to break down water into hydrogen and oxygen:

2H2O + energy –> 2H2 + O2

With an 80% efficient electrolysis unit, it takes about 50 kWh of electricity to create 1 kg of hydrogen. The hydrogen could then be used in a fuel cell or converted gas engine to generate electricity.  This is great except for the “detail” of storing the hydrogen.  (What to do with the resultant oxygen is another detail.  It can be bottled and sold, or simply released into the air.)  Most people suggest compressing the hydrogen possibly to a liquid form, but that takes considerable energy, and the result is not economical for the production of electricity.  Not compressing it takes too much storage and pumping to move it around.

Multiple methods for storing the generated hydrogen as hydrides, e.g., magnesium hydride, MgH2, have been developed over the years.  For MgH2, they involve grinding the magnesium in a hydrogen environment at pressure (15 Bars or so), at high temperature (in excess of 300°C), and in the presence of a catalyst. (Various catalysts have been proposed, including MgH2 itself.) This process gives an initial supply of MgH2 for use in an energy storage cycle.

The folks at Safe Hydrogen mix the resulting MgH2 into a “slurry” by adding light mineral oil and some dispersants. The dispersants aid in keeping the MgH2 particles from agglomerating and help to stabilize the slurry. The mineral oil helps to protect the MgH2 particles from inadvertent contact with moisture in the air and provides the liquid medium for the slurry.  The resulting slurry is then cooled to room temperature for stable storage.  This process is described in their patent US 2010/0252423 A1. The slurry looks a lot like thick paint.  Here’s a photo from one of the Safe Hydrogen papers, links to which can be found on their web site.70 Percent MgH2 Slurry

At ambient temperatures, the slurry is stable, not highly flammable, and can be pumped, transported, and stored in standard tanks and trucks.  It has stored energy by kg or by liter (L) as high as [0] 3.9 kWh/kg and 4.8 kWh/L.  Compare this to gasoline which has 1.8 KWh/kg of practical energy storage within 12.8 kWh/kg of total theoretical energy storage. [1]

The slurry can be converted back to hydrogen and recycled as follows:

 

The first technique is to add water to release the hydrogen:

MgH2 + 2H2O –> Mg(OH)2 + 2H2

The resulting byproduct, magnesium hydroxide, Mg(OH)2, is, in its pure form, a white power, and when mixed with water forms a suspension that is essentially the same as “Milk of Magnesia.”  Recycling the Mg(OH)involves several steps:

  1. The separation of the oils from the byproduct slurry for reuse.
  2. The calcination of Mg(OH)2 to MgO by heating to around 300°C,

Mg(OH)2 –> MgO + H2O,

  1. The electrolytic reduction of the resulting magnesium oxide using the “solid-oxide • oxygen-ion-conducting membrane (SOM) process developed atBostonUniversity[2].

2MgO –> 2Mg + O2

  1. The hydriding of the Mg obtained in the preceding step and H2 (obtained by additional electrolysis of water) to MgH2, is done by mixing magnesium powder with magnesium hydride powder and hydrogen at about 300°C and 10 Bar.
  2. The production of new slurry from the MgH2 and the recovered oils.

In this process, slurry is pumped into a stream of hot water to mix the slurry with the water. The mixed stream then flows into a reaction chamber where the bulk of the reaction takes place, releasing hydrogen. Several injections take place in quick succession to take advantage of the heat from the previous reaction. The byproducts of this reaction collect in the bottom of the reactor. Periodically, the reaction chamber is flushed by filling it with water to recover the oils from the slurry. The water level is reset and the injections resume. Also periodically, the byproducts are moved from the bottom of the reactor to the byproduct separation chamber. In the byproduct separation chamber, the water is filtered from the solid byproducts and returned to the water storage container. One can readily hydride magnesium by mixing magnesium powder with magnesium hydride powder and hydrogen at about 300°C and 10 Bar [This step is described in detail in the above cited patent.].  The heat produced by the electrolysis process was estimated to be enough to satisfy the heating requirements of the system in continuous operation.

Effort needs to be performed to condense magnesium into a powder from the vapor produced by the SOM process. To minimize the fire hazards associated with magnesium powder, the powder should be immediately hydrided, and the resulting magnesium hydride should proceed directly into a slurry production process.

According to Andrew McClaine, CTO at Safe Hydrogen, “We are currently scaling up from a laboratory scale of 2 kWth to a scale of 20-40 kWth.”  In other words, this technology needs a lot of development before it can scale to utility sized operations, which would require tens to hundreds of MWh of energy stored.  Note that 10MWh would take about 2,000 liters of MgH2 slurry.  Safe Hydrogen’s estimates on cost seem reasonable, but the process is complex with lots of variables.  It usually takes a decade for such technology to become cost effective and mass produced.

McClaine also points out that there is another technique to release hydrogen from the slurry.  It is to heat it to around 280°C.  This releases some of the hydrogen rather efficiently and allows the slurry to be replenished later by adding hydrogen from the electrolysis process.  Apparently this process can be repeated efficiently approximately 100 or so times, before the complete first process needs to be performed.  Safe Hydrogen is working on optimizing this combined technique and on increasing the number of cycles from 100 to potentially 1000 or so.

Refining all these techniques may well produce a commercially viable product, even at a scale of 20-40 kWth.  Safe Hydrogen seems to think so.  It appears that smoothing out the energy produced from wind farms may have an earlier commercial product than transporting the slurry and producing hydrogen elsewhere. [3]

-gayn

[0] Certain proposed lower cost processes will produce a lower energy density of the slurry.  Safe Hydrogen is experimenting with various processes.  Personal communication Andrew McClaine, 8/28/2011.

[1] Journal of Physical Chemistry and Letters

[2] Solid-Oxide Oxygen-Ion-Conducting Membrane (SOM) Technology For Production Of Magnesium Metal By Direct Reduction Of Magnesium Oxide, D. E. Woolley and U. Pal, Department of Manufacturing Engineering, Boston University, Boston, MA 02215, G. B. Kenney. ElMEx, LLC, Medfield. MA 02052

[3]  Storing Wind Energy as Hydrogen, David Anthony and Ken Brown, reprinted in Green Economy Post, August 15, 2011.

Metal-air Batteries

2011/08/29

Metal-air Batteries

As discussed in a previous post, wind and solar generators need energy storage mechanisms for a variety of reasons.  This post discusses some promising new battery technology, rechargeable metal-air batteries.  Single use metal-air batteries have been around for years.  The new technology here is to design them so that they are efficiently rechargeable.

There are three sizes of applications to think about:  little button batteries for electrical gadgets (usually single use), batteries for electric vehicles, and batteries to smooth the energy generation of utility-sized wind and solar generators.

The current commercial state-of-the-art, Li-ion batteries, which are used in today’s electric and hybrid vehicles, are really heavy and they cost a lot!  They have some other problems.  First the range today of an EV is 50-200 miles whereas we’re used to say 350 miles.  Second, the charging time is in the order of 7 hours, whereas we’re used to 3-4 minutes to refill a car’s gas tank.  These problems, of course, get worse when scaled to utility sized units; hence, the industry is driven to a new generation of battery.

Metal-air batteries use oxygen directly from the air, which allows for higher total energy density due to unlimited cathode capacity.  This definitely will reduce the weight.  The possible metals investigated by the industry are Zn, Fe,Al, Mg, and Li, with zinc (Zn) and lithium (Li) being the most frequently discussed.  One model predicts that the overall theoretical energy density of polymer electrolyte Li-air battery could be as high as 2790 Wh/kg and 2800 Wh/L, which is comparable to gasoline-air combustion engines [1].  The following table comes from Chemical and Energy News, 11/22/10.  Note that Li-air batteries have 10-11 times the energy storage potential that Li-ion batteries do (per weight).  Zn-air batteries have about four times the energy storage potential as Li-ion batteries.  The potential or theoretical numbers are computed from the energy released from the metal assuming total oxidation from the oxygen.  It is clearly much greater than the energy released from the corresponding metal-air battery today; although this battery technology will improve over time.

This would address the weight and energy storage, but rechargeability is a problem.

Older lithium-air batteries do not have long lasting bi-functional cathodes where the oxygen reduction and evolution both take place.  Effective catalysts are needed to reduce the byproducts of the discharge, such as Li2O2 and Li2O.  These are not soluble in current electrolytes and eventually clog the pores of the cathodes, seizing the cell.  Membranes between the anode and cathode also can clog.  Similar problems occur for older zinc-air and other older metal-air batteries.  Thus to make effective (i.e. having a high rates of oxygen reduction and evolution) rechargeable metal-air batteries, new materials for cathode, catalysts for both the reduction and evolution cycles, electrolytes, and membranes are needed.

There are more problems uncovered by current research on Li-air.  Li-air batteries require more voltage to charge them than one obtains when using them.  This ratio is called “energy efficiency”, and most metal-air batteries today have energy efficiency around 60%.

Another problem is the discharge rate.  The reaction between lithium and oxygen in today’s Li-air batteries proceeds too slowly to generate significant current.  It is a little impractical to gang them up to get adequate current.

Next cost.  While current research tends to favor the performance of lithium-air batteries, zinc is a far more abundant metal than lithium, and hence the cost of zinc-air batteries could be significantly lower than that of lithium-air batteries.  At least two companies are headed in this direction with their zinc-air technologies.  One, ReVolt Technology, has a patented [2, 3] mixture of materials for the anode, cathode, catalyst, electrolyte, and membrane, and the other company, EOS Energy Storage, has a patent-pending mixture.  Both believe that their technologies can scale to utility sized batteries, i.e. batteries with tens to hundreds of MW/h of energy stored, and both believe that they can address the EV market (which would give them the scale to keep the price down at the utility level.)  In other words, both believe they have solved the above mentioned problems with today’s Li-air batteries.  The industry continues with considerable research being done by various organizations.  These organizations all hold a wide variety of related patents.  Lawyers will do well when the industry starts to shake out!

Of course, just as an array of wind or solar generators is needed to produce utility sized amounts of energy, an array of batteries is needed with a capacity sufficient to smooth out the peaks and valleys of the generated electricity.  E.g. the wind might not blow or the sun might not shine for several days in a row.  Allowing for these extremes could drive the cost of the batteries unacceptably high.  As a second level of backup, natural gas generators could also be part of the system, but one would expect very little natural gas to be burned over the course of the year with reasonably sufficient batteries.

-gayn

[1] J.P.Zheng et al, J.Elec.Chem.Soc.155(6)A432-A437 (2008)

[2] Patent US 2007/0166602 A1, “Bifunctional Air Electrode”

[3] Patent US 2008/0096061 A1 “Metal-Air Battery or Fuel Cell”

CPV Environmental Impact

2011/08/22

 

Today I read the paper “An Assessment of the Environmental Impacts of Concentrator Photovoltaics and Modeling of Concentrator Photovoltaic Deployment Using the SWITCH Model” June 2011 by Dr. Daniel Kammen of the Renewable and Appropriate Energy Laboratory at UC Berkeley and his Ph.D. students James Nelson, Ana Mileva, and Josiah Johnston.  [cf. www.rael.berkeley.edu]  It’s not too long (25 pages) and is packed with information.  I recommend it.  The review/summary here contains my comments.

The paper discusses, in the context of other forms of energy generation, the comparative environmental impact of CPV.  The short answer, if you don’t want to read any more, is that CPV has comparatively minimal environmental impact, and in particular is slightly worse than PV due to the former’s tracking system (see below) and a tiny bit better than CSP due to the latter’s cooling and water use.  Of course, oil, coal, and even natural gas are bad environmentally for a variety of reasons, and thankfully the paper doesn’t harangue about this too much.

The report considers three Life Cycle Assessment (LCA) phases: (1) fabrication and deployment of the energy generation facility, (2) energy production and maintenance, and (3) recycling and disposal at end of life of the facility.  Discussed in this context are the environmental issues of energy, emissions, water use, and land use.  Also considered in this report is the Energy Pay-Back Time (EPBT) which is the time (in years) that it takes to generate the net energy (roughly the energy generated minus energy used) in Phase 2 to be equal to the energy used in Phases 1 and 3, i.e. in creation and disposal of the generation facility.  Heretofore, this number has been estimated in the 3-15 year range; however, this report estimates it as less than 1 year for both PV and CPV. Caution:  one review of this paper that I read has challenged the logic and data used by the authors in their calculations of EPBT.  The authors point out that EPBT calculations are also sensitive to the geographic location of the generation facility, and hence have significant variance.

Green House Gas (GHG) emissions for Phase 1 are considered.  They are highest for large tracking systems such as CSP and CPV that use a lot of GHG-intensive steel in their structures. Such steel is heavy and incurs greater shipping costs in Phase 1.  Not only can the designs be improved to reduce the amount of steel used, but such steel could be manufactured where a large portion of the steel production energy came from solar or at least renewable energy sources.

Water use during Phase 1 is difficult to estimate due to lack of data on recycling, CPV use is estimated at 2 times that of PV.  Water use during operations, Phase 2, is significant since the large sites tend to be in water constrained regions.  In any case, for PV and CPV, water is only used for washing as opposed to CSP where it is used for wet cooling.  If dry cooling, i.e., air cooling, is used in a CSP plant, then water use is reduced 90% and is relegated to washing and steam production.

When mining, transportation, and disposal of non-renewable fuels are taken into consideration, the land use per GWh of renewable and non-renewable generations turns out to be comparable.  Hydroelectric and Wind are several times higher, and rooftop PV systems, where the land is already used by the building, are several times less. CPV has an advantage in land use over PV and CSP.  In addition, most large CPV and CSP systems are pole mounted, allowing potential reuse of most of the land under the arrays for plants and small animals.  Such use, it is noted by the authors, is not currently common; although CPV vendors seem to be leading here. Environmental impact studies should (by law) address impact on flora and fauna in the region; however, such studies can get around this by stating the impact is minimal in the large.

The authors also model CPV deployment in the southwest area covered by the Western Electricity Coordinating Council (WECC) by starting with around 1000 existing installations of various types and varying the mix of 10,000 additional renewable and conventional installations.  Their model runs thousands of wind and sun conditions on an hourly basis to meet the forecasted needs and to minimize the cost of generation, storage, and transmission.  The model uses NREL’s System Analysis Model (SAM) to get various costs of operation and maintenance.  At this point the paper is somewhat unclear as to just how the mix of the additional 10,000 generation types is determined.  It appears that CPV sites are preferentially added and CSP sites are not considered by this modeling.  That said, the conclusions are:

  • It would be economical to install between 12 and 43 GW of CPV by 2030 in the United States Desert Southwest
  • Including CPV allows for deeper CO2 reductions in the electric power system
  • CPV displaces natural gas generation on the margin
  • Strong carbon policy (e.g. charging $40/ton for CO2 generation) increases the deployment of CPV

Analyzing several of the graphs in this paper, it also appears that with the authors’ investment assumptions, the WECC area will need to increase its use of natural gas to cover peak usage.

In summary, I wish there were more studies like this.  Indeed, the authors’ model could be run many times over with differing investment assumptions to generate additional fascinating information.

-gayn

Concentrated Solar Power (CSP)

2011/08/19

Concentrated Solar Power (CSP)

The basic idea behind Concentrated Solar Power (CSP) is dead simple, especially if you liked to start fires with a magnifying glass when you were a kid:  Focus a lot of sunlight to gather heat, and use this heat to generate electricity.

There are several ways to do this.  A simple way, to get our mental juices flowing, would be to use the heat source to boil water to generate steam, and then use a steam engine to drive a generator to produce electricity.  While simple, this approach turns out to have a few problems, e.g., needing a water supply in the middle of a desert and the fact that steam is not a good energy storage medium.  Thus, sometimes this approach is used as a “back end” of a more complicated system, where one fluid is heated to a very high temperature, and this hot fluid is used to create steam for a steam engine.  This is discussed more below.

A vastly better approach is to focus the sun’s rays to create a very hot liquid, and to use it to heat a Stirling Engine to generate electricity.  The very hot liquid stays external to the Stirling Engine.  There is a nice article on Stirling Engines in Wikipedia here. Like steam engines, they are external combustion engines as opposed to internal combustion engines as are gasoline and diesel engines.  Here’s a picture from the Wikipedia article.  Since WordPress doesn’t support motion in gif files, you’ll have to click on the picture to see motion.

Stirling Engine

For this simple Stirling Engine, there is only one cylinder, hot at one end and cold at the other. A loose fitting displacer shunts the internal fluid between the hot and cold ends of the cylinder. A power piston at the end of the cylinder drives the flywheel, which in turn drives the generator.  In our case, our external hot fluid replaces the external “fire” in this picture.

Let’s break a CSP system down a little more.  There are several ways to “focus” the sunlight.  First there are three dimensional parabolic dishes that focus the light essentially onto the hot side of the Stirling Engine.  Here are some images taken from Google’s image collection (Click on them to see their original source.):

Above there is a single CSP with one Stirling Engine at the focal point, and below an array of such CSPs.

Note in this array there are multiple Stirling Engines. While more expensive, this is goodness, because the array keeps working in the presence of even multiple engine failures.

Another approach is for movable mirrors (heliostats) to reflect the sunlight to a “point” on a tower.  The mirrors are usually rather flat parabolic mirrors so that their focal point is quite far away – near the top of the tower.

In this tower/heliostat approach, the external fluid is heated at the top of the tower and piped down to a Stirling Engine or to a steam engine.  After it has been used for heating, the cooled external fluid is piped back to the tower to be reheated.

A second form of CSP uses a two dimensional parabolic trough:

Along the line of parabolic foci is a thermal collector pipe that not only serves as an “oven” for the heated fluid, but it also transports the heated fluid to a central generating facility.  Below is such a facility in Abu Dhabi, at one of the world’s largest CSP plants.

A third way to focus sunlight is with a Fresnel lens or mirror.  Recall these are used by many CPV systems as well.  In the lens case the thermal collector pipe is below the lens.  For Fresnel mirrors, it is above the mirrors. Here’s a photo:

The company Areva, has a fourth combination-type of setup.  Here are a couple photos from their web site:

The blue trough is a parabolic trough focusing the light onto the overhead “compact linear Fresnel reflector” which in turn focuses the light onto a thermal collector pipe.  Here’s a close-up photo of that:

To maximize efficiency, all CSP systems need tracking so that they “point” towards the sun.  The trough and Fresnel systems need only single axis tracking, while dual axis tracking is needed for tower/heliostat CSPs and also for parabolic dish arrays.

One potential problem CSP systems have is glare.  A system under construction in Cloncurry, Australia was scrapped in November 2010 due to concerns about reflective glare in an urban environment.

The next part of a CSP system is the external heating fluid, which is typically some sort of liquid salt, although other materials including super-heated steam and graphite have been used. The molten salt is a mixture of 60 percent sodium nitrate and 40 percent potassium nitrate, commonly called saltpeter. New studies show that calcium nitrate could be included in the salts mixture to reduce costs and with technical benefits. This salt melts at 220 °C (430 °F) and is kept liquid at 290 °C (550 °F) in an insulated storage tank.   The main feature of liquid salt is that it can be heated to very high temperatures, in excess of 500 °C, and at this heat, it can be used to drive a Stirling Engine or generate steam for many hours after the sun goes down.  This time of course depends on the initial temperature, the quantity of liquid, the insulation, and the efficiency of the Stirling or steam engine used.  Typically current systems continue to generate significant amounts of electricity for 7 to 8 hours after the CSP stops heating the salt.  This isn’t quite enough for 24 hour operation, and some utility companies set up “hybrid” systems where the load at night is met with a gas fired generator.

These systems are improving, and the 19.9 MW CSP system in Andasol, Spain generated 24 hours of continuous electricity in July 2011 using a molten salt storage system.  This design should average 20 hours per day of electrical power generation, hitting 24 hours on particularly sunny summer days.  This plant has a field of 2650 heliostats that focus sunlight on a central tower. Here’s a photo:

Future posts will compare CSP systems with CPV systems for efficiency and cost effectiveness.

-gayn

Storing Energy from Solar Arrays

2011/08/11

Storing Energy from Solar Arrays

Even in the case there is heavy daytime use for solar power, e.g. air conditioning, there will always be a need for power when the sun can’t provide it.  Ideally, the solar generation system will be over-specified so that peak usage can be handled, in which case there is a need to store the excess energy for “after sun” hours.  In addition, peak energy consumption is in the afternoon, and solar arrays are effective in the morning hours, which might generate excess power in the mornings.

Now the best “storage” device is usually the grid.  Dump your excess energy onto the grid so that someone else can use it, and then use the grid whenever (day or night) your solar power isn’t adequate.

The problem is that the grid has only minuscule renewable storage capacity – most of it behind dams that contain hydro-electric generators. Thus “storing” excess energy on the grid assumes that someone can use it at essentially that moment.  Utility companies try to shape demand by turning on and off not only hydro-electric generators, but also (ugh) fossil fuel generators and by adjusting the various components of energy pricing.  For the excess producer of solar or other renewable energy, this means that putting electricity onto the grid at non-peak times, may provide low or even negative prices for that energy.

That said, what if you can’t (or don’t want to) dump your excess power onto the grid for some reason?  Here are some thoughts to be elaborated on in subsequent postings:

  • Store the energy as heat, e.g. as hot water.  This can be done in hot water tanks, or even in a swimming pool.  A more sophisticated method is to store the heat in a heat sink such as molten salt (very hot), which can be reused in various ways.  There are various salts besides NaCl – common table salt – but the idea is to heat the salt to over 500 degrees Celsius.  Well insulated, liquid salt can keep 90% of its heat for over 24 hours.
  • Storing heat in gravel has merit.  Isentropic’s Pumped Heat Electricity Storage (PHES) system is based on the First Ericcson cycle and uses a heat pump to store electricity in thermal form. The storage system uses two large (7m high  x 8m in diameter) containers of gravel, one hot (500C) and one cold (-150C) with a heat pump machine between them. Electrical power is input to the machine which compresses/expands air to 500C on the hot side and -150C on the cold side. The air is passed through the two piles of gravel, where it gives up its heat/cold to the gravel. In order to regenerate the electricity, the cycle is reversed with a round trip efficiency of 70-80 percent. The temperature difference is used to run the system as a heat engine.
  • Compressed Air Energy Storage (CAES) is very interesting; however,  don’t think of a tank of compressed air, but rather think of an underground cavern filled with it.  In the middle of the night when the price of electricity is low, utilities can run compressors and pump air into a cavern at around 750 psi.  When the price of electricity goes up, the compressed air is then used to power a turbine generator. Often this portion of the design is supplemented by the use of natural gas either to heat the air or to mix with the pressurized air to burn.  Investigations by EPRI indicate that up to 80 percent of the U.S. has geology suitable for CAES. A single 300 megawatt CAES plant would require 22 million cubic feet of storage space — enough to store eight hours’ worth of electricity. Ridge Energy has a nice diagram here. Whether CAES systems can be commercially viable at a utility level remains to be seen.  One negative argument is here.
  • Store it as kinetic energy, e.g. in a fly wheel.  Well, there are horror stories of large fly wheels disintegrating with hunks flying around causing much damage.  Not a pretty picture.  Fly wheels in general lose a lot of energy to friction, and really are only good to transition use from one source of energy to another.  Lots of small fly wheels are too expensive to maintain. The company Velkess (founded 2007) is developing new flywheel technology.  Keep an eye on them.
  • Pump water up hill.  This isn’t as silly as it sounds, especially if what is needed is water pressure.  Think of those old fashioned city water tanks, which are decidedly useful. On a large scale, filling a reservoir that was above a hydro-generation facility would be a good idea.  For example, the TVA’s Racoon Mountain pump-up facility has been operating for a number of years.  Some estimate that about 10% of the hydro-electric dams are suitable for this.
  • Freeze water.  If you don’t have enough use for ice, e.g., for cooling, then run the freezer during the day and let it “coast” at night.  Some commercial refrigerators have logic for this, expecting both power and usage to be nil at night and to start up in the morning.  Even when commercial electricity costs less at night, this is a win for solar.  For wind, making ice at night and using it to cool a building during the day is done by CALMAC.
  • Use electrolysis to make hydrogen, then burn the hydrogen later to make electricity.  This approach is costly, and it takes considerable energy to compress the hydrogen into a liquid form so that it can be shipped or stored easily.
  • A more sophisticated version of the preceding is to use electrolysis to make MgH2 – magnesium hydroxide.  This is a relatively stable liquid, and the hydrogen can be released either by mixing the MgH2 with water or by heating it.  The released hydrogen can then be burned to make electricity.  The byproduct after adding water, Mg(OH), is essentially Milk of Magnesia. which can be recycled to recover the magnesium and start the process all over again. In addition to storing energy in the form of hydrogen, this “liquid energy” and its byproduct can be transported easily in trucks, boats, etc..
  • Of course batteries are the standard answer, and battery technology is getting better every year.  Here, you get a little extra mileage bypassing the inverter(s) and using DC appliances, but I’m not sure what the trade-offs are.  For example, using LED’s for DC lighting would be a good idea.  It is pretty easy to bypass the power supplies in most computer equipment.  DC fans would work.  But the cost of DC ovens, air conditioners, etc. is probably prohibitive.  At the scale of utilities, EOS Aurora and Revolt Technology both claim to have rechargeable Zinc-Air batteries that will scale hold 6 MWh each.  Zinc-air batteries aren’t as good as Lithium-air, but they are vastly cheaper.  The battery researchers are lately hot on Vanadium Redox batteries which you can read about here.

As I’ve thought about putting solar arrays on my house’s rooftop, some of the above ideas may work for me, e.g. heating water, and getting the freezer and refrigerator extra cold during the day.  On the other hand, I’ll also connect to the grid and take the pittance that the electric company gives me for my “donations.”

Finally, for you wind farm fans, most of these ideas apply – with the exception of storing liquid salt.  Not that a wind mill can’t make liquid salt, but solar arrays essentially start there.

-gayn

Killing birds

2011/08/03

Today’s (8/3/11) LA Times (page AA1) reports that federal authorities are investigating the deaths of golden eagles at the LA Department of Water and Power’s Pine Tree Wind Project in the Tehachapi Mountains.  Environmentalists have long complained about damage to fauna caused by these huge wind generators – from birds to underground fauna.  Things do get dicey, however, when endangered species such as golden eagles  and bald eagles are getting (literally and figuratively) “whacked.”  These recent deaths are not the first, and apparently not much noise has been made when 1500+ birds are killed each year at the Pine Tree facility.  These deaths mostly are “ordinary” migratory song birds, quail, and meadowlarks.

Now I’m not being an apologist, but it is important to understand that over 400,000 birds are killed each year nationwide by encounters with buildings, utility towers, cars, etc.  My own house kills a couple of small birds each year as they fly into our picture windows.

Two questions arise that call for answers: First, are the bird deaths due to wind generators “acceptable” as compared to the already acceptable deaths caused by other structures?  And second, if not, especially if not for endangered species, then what can we do technologically to mitigate these deaths?

It’s probably impractical to enclose a wind farm (and especially a single generator) in a huge chicken-wire fence, but ideas that come to my mind are whistles, sirens, recorded bird screeches, and strobe lights.   Dang if I know, but let’s do something!

-gayn

Shading rooftops with solar panels

2011/07/26
Another well-known argument for rooftop solar panels is that they help cool buildings through the shade that they provide.  Anthony Dominguez, Jan Kleissland Jeffrey C. Luvall of UCSD and NASA recently published a paper in Solar Energy “Effects of solar photovoltaic panels on roof heat transfer” in which they claim (via minor testing and a heat flux model) a 5.9 kWh m−2 (or 38%) reduction in annual cooling load as well as some (not quantified) reduced thermal stress on the roof.  The effect of the wind circulating under the panels is part of this energy savings.
The panels also reduce warming the building in the winter during the day, but the paper claims there is a compensating reverse “blanket” effect at night.  To me, it is a little counter-intuitive that the blanket effect would totally compensate for the loss of winter warming, especially taking into account the additional cooling effect of wind blowing under the panels.
The bottom line is a quantification of the additional energy savings, which they calculate at about 5%.  This 5% was for San Diego, but I would expect similar savings for most regions that use solar panels.
They used PV panels which I assume were fixed inclination.  I would imagine CPV panels with a dual axis tilt mechanism would yield slightly better shading effects, but the extra energy savings might be negligible.
It would be nice to have a statistical study of this extra savings over a large number of buildings and a large number of geographic regions.
-gayn